The dystrophin-associated protein complex (DAPC) links the cytoskeleton to the extracellular matrix and is necessary for maintaining the integrity of the muscle cell plasma membrane. The core DAPC consists of the cytoskeletal scaffolding molecule dystrophin and the dystroglycan and sarcoglycan transmembrane subcomplexes. The DAPC also serves to localize key signaling molecules to the cell surface, at least in part through its associated syntrophins (Brenman, et al. (1996) Cell. 84: 757-767; Bredt, et al. (1998), Proc Natl Acad Sci USA. 95: 14592). Mutations in either dystrophin or any of the sarcoglycans result in muscular dystrophies characterized by breakdown of the muscle cell membrane, loss of myofibers, and fibrosis (Hoffman, et al. 1987. Cell. 51: 919; Straub and Campbell (1997) Curr Opin Neurol. 10: 168). Moreover, mutations in the extracellular matrix protein laminin-α2, which associates with the DAPC on the cell surface, are the basis of a major congenital muscular dystrophy (Helbling-Leclerc, et al. (1995) Nat Genet. 11: 216).
The α-/β-dystroglycan subcomplex forms a critical structural link in the DAPC. The transmembrane β-dystroglycan and the wholly extracellular α-dystroglycan arise by proteolytic cleavage of a common precursor (Ibraghimov, et al. (1992) Nature 355: 696; Bowe, et al. (1994) Neuron 12: 1173). The cytoplasmic tail of β-dystroglycan binds dystrophin, while the highly glycosylated, mucin-like α-dystroglycan binds to several ECM elements including agrin, laminin, and perlecan (Ervasti and Campbell, (1993) J Cell Biol. 122: 809; Bowe, et al. (1994) Neuron. 12: 1173; Gee, et al. (1994) Cell 77: 675; Hemler, (1999) Cell 97: 543). This binding to matrix proteins appears to be essential for assembly of basal lamina, since mice deficient in dystroglycan fail to form these structures and die very early in development (Henry, M. D. and K. P. Campbell. 1998. Cell. 95: 859). β-Dystroglycan can bind the signaling adapter molecule Grb2 and associates indirectly with p125FAK (Yang, et al. (1995) J. Biol. Chem. 270: 11711; Cavaldesi, et al. (1999), J. Neurochem. 72: 01648). Although the significance of these associations remains unknown, these binding properties suggest that dystroglycan may also serve to localize signaling molecules to the cell surface.
Several lines of evidence suggest that dystroglycan may also function in neuromuscular junction formation, in particular, in postsynaptic differentiation. For purposes of clarity, the components of the neuromuscular junction are summarized here. The major structural features of the neuromuscular junction (NMJ) or nerve-muscle synapse are the pre- and post-synaptic specializations of the motor neuron and muscle, respectively, the intervening synaptic basal lamina, and the specialized Schwann cell cap (Salpeter, et al (1987) The Vertebrate Neuromuscular Junction. New York, Alan R. Liss.). The presynaptic apparatus is marked by ordered arrays of synaptic vesicles, a subset of which are poised to fuse with the plasma membrane at the active zones, and release acethylcholine that is recognized by acetylcholine receptors (AChRs) on the muscle, and ultimately results in electrical activation and contraction of the muscle (Heuser, et al (1981) J. Cell Biol. 88: 564). Immediately across the 50 nm synaptic cleft from these zones are the crests of the postjunctional folds. These crests bristle with Acetylcholine receptors (AChRs), which can reach densities of >10,000 molecules/μm2 (Fertuck, et al (1976) J. Cell. Biol. 69: 144). The localized and tightly regulated secretion of acetylcholine into the narrow synaptic cleft, coupled with the high AChR density in the postsynaptic membrane, ensures rapid and reliable synaptic transmission between neuron and muscle. Perturbations of these specializations, such as the decrease in the number of functional AChRs seen in myasthenia gravis, can lead to debilitating and often fatal clinical outcomes (Oosterhuis, et al (1992) Neurology & Neurosurgery 5: 638).
The synaptic basal lamina (SBL) is interposed between the pre- and post-synaptic membranes and contains molecules important for the structure, function, and regulation of the neuromuscular junction (Bowe, M. A & Fallon, J. R., (1995) Ann. Rev. Neurosci. 18: 443; Sanes, et al. (1999) Ann. Rev. Neurosci. 22: 389). It consists of a distinct set of extracellular matrix molecules including specialized laminins, proteoglycans and collagens (Hall, et al (1993) Neuron 10: (Suppl.) 99). The SBL also contains molecules essential for the regulation of synaptic structure and function including AChE, neuregulins, and agrin. The SBL thus serves both as a specialized structure for maintaining the localized differentiation of the synapse as well as a repository for essential regulatory molecules.
The molecular composition of the postsynaptic membrane is known in considerable detail. As noted above, the most abundant membrane protein is the AChR. The cytosolic AChR associated protein rapsyn (formerly known as the 43 kD protein) is present at stoichiometric levels with the receptor and is likely to form a key link between the cytosolic domain of the AChR and the cytoskeleton (Froehner, et al (1995) Nature 377: 195; Gautam, et al. (1995) Nature 377: 232). The postsynaptic membrane is also enriched in erbB2-4, some or all of which serve as neuregulin receptors (Altiok, et al. (1995) EMBO J. 14: 4258; Zhu, et al. (1995) EMBO J. 14: 5842). AChR and other molecules are essential for nerve-muscle communication. The cytoskeletal elements can be broadly grouped into two subsets. Dystrophin and utrophin are members of the dystrophin-associated protein complex, or DAPC, and are linked to the synaptic basal lamina via the transmembrane heteromer α-/β-dystroglycan. The postsynaptic cytoskeleton is also enriched in several focal adhesion-associated molecules including α-actinin, vinculin, talin, paxillin, and filamin (Sanest et al (1999) Ann. Rev. Neurosci. 22: 389). The latter proteins probably communicate, directly or indirectly, with the extracellular matrix through integrins, some of which are enriched at synapses (Martin, et al. (1996) Dev. Biol. 174: 125). Actin is associated with both sets of cytoskeletal molecules (Rybakova et al. (1996) J. Cell Biol. 135: 661; Amann, et al. (1998) J. Biol. Chem. 273: 28419-23; Schoenwaelder et al. (1999) Curr. Opin. Cell. Biol. 11: 274). The functions of these specialized sets of proteins are considered below.
α-Dystroglycan binds the synapse organizing molecule agrin (Bowe, et al. (1994) Neuron. 12: 1173; Campanelli, et al. (1994) Cell. 77: 663; Gee, et al. (1994) Cell. 77: 675; Sugiyama, et al. (1994) Neuron. 13: 103; O'Toole, et al. (1996) Proc Natl Acad Sci USA. 93: 7369) (reviewed in Fallon and Hall, (1994) Trends Neurosci. 17: 469), and β-dystroglycan binds to the AChR-associated protein rapsyn (Cartaud, et al. (1998) J Biol Chem. 273: 11321). Further, agrin-induced AChR clustering on the postsynaptic membrane is markedly decreased in muscle cells expressing reduced levels of dystroglycan (Montanaro, et al. (1998) J Neurosci. 18: 1250). The precise role of dystroglycan in this process is unknown. Currently available evidence suggests that dystroglycan is not part of the primary agrin receptor, but rather may play a structural role in the organization of postsynaptic specializations (Gesemann, et al. (1995) Biol. 128: 625; Glass, et al. (1996) Cell. 85: 513; Jacobson, et al. (1998) J Neurosci. 18: 6340).
Another molecule that plays an important role in neuromuscular junction formation is the tyrosine kinase receptor MuSK, which becomes phosphorylated in response to agrin. However, agrin does not bind to MuSK and it is unclear how agrin stimulates MuSK. The existence of a co-receptor had been suggested. Activation of MuSK by antibody cross-linking is sufficient to induce the clustering of AChRs on cultured myotubes (Xie et al. (1997) Nat. Biotechnol. 15:768 and Hopf and Hoch (1998) J. Biol. Chem. 273: 6467) and a constitutively active MuSK can induce postsynaptic differentiation in vivo (Jones et al. (1999) J. Neurosci. 19:3376). However, MuSK phosphorylation is necessary but not sufficient for agrin-induced AChR clustering.
The realm of dystroglycan function ranges far beyond muscle. As noted above, mice defective in dystroglycan die long before muscle differentiation. In a surprising development, a-dystroglycan in non-muscle cells has been shown to function as a receptor for Lassa Fever and choriomeningitis fever viruses (Cao, W., et al., 1998, Science. 282: 2079), and on Schwann cells as a co-receptor for Mycobacterium leprae (Rambukkana, et al. (1998) Science. 282: 2076). Dystroglycan is also abundant in brain, but its function there is not understood (Gorecki, et al. (1994) Hum Mol Genet. 3: 1589; Smalheiser and Kim (1995) J Biol Chem. 270: 15425):
α-Dystroglycan is comprised of three known domains. An amino-terminal domain folds into an autonomous globular configuration (Brancaccio, et al. (1995) Febs Lett. 368: 139). The middle third of the protein is serine- and threonine-rich, and is highly glycosylated (Brancaccio, et al. (1997) Eur J Biochem. 246: 166). Indeed, the core molecular weight of α-dystroglycan is ˜68 kDa, but the native molecule migrates on SDS-PAGE as a polydisperse band whose size ranges from 120-190 kDa, depending upon the species and tissue source (Ervasti and Campbell (1993) J Cell Biol. 122: 809; Bowe, et al. (1994) Neuron. 12: 1173; Gee, et al. (1994) Cell. 77: 675; Matsumura, et al. (1997) J Biol Chem. 272: 13904). Glycosylation of α-dystroglycan, probably in this middle third, is essential for its laminin- and agrin-binding properties.
While it is clear that dystroglycan and the DAPC play crucial: roles in a variety of processes in muscle as well as in other tissues, the underlying mechanisms remain obscure.